A cellular wireless communication system can be configured of a multi-stage beam forming system such as a beam division multiple access (BDMA) system. As an example, multi-stage beam forming can include digital beam forming and analog beam forming using a directional antenna. In a typical communication environment, the analog beam forming is operated prior to the digital forming, and thus a status of a channel in which the digital beam forming is performed is affected by the analog beam forming which is performed prior to the digital beam forming. Accordingly, in the digital beam forming, it is necessary to consider a design and an operational condition of the analog beam forming which is a front end of the digital beam forming.
Like in the typical communication system, the digital beam forming of the multi-stage beam forming system is operated based on channel information. Accordingly, a receiver reports information on a change of a channel environment according to an operation of the analog beam forming to a transmitter.
Channel estimation for the digital beam forming is very limited due to its characteristic. For example, when the number of radio frequency chains of a digital stage is small compared to the number of antennas of an analog stage, it is very difficult to obtain channel information on all antennas of the analog stage. In this circumstance, the obtained channel information can limit an operation of front-end beam forming, and also cannot provide a statistical characteristic of the channel environment for an operation of the digital stage.
For the beam forming of the digital stage, a channel information feedback configuration based on a code book shared by the transmitter and the receiver may be used. Here, a term “code book” means a set of candidates of pre-coding matrixes used for the beam forming of the digital stage. It is necessary to adaptively design the code book so as to adjust the code book to the channel environment. However, in the conventional art, because a multi-stage beam forming environment is not considered, the design of the code book is not associated with the operation of the front-end beam forming such as the analog beam forming. Particularly, there is a problem in that the conventional art cannot provide the adaptive design of the code book to which the operational condition of the beam forming is applied.